1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to reproducing information using a semiconductor probe, and more particularly, to a method for separating an information signal generated by a variation in an electric field of a medium from a noise signal generated by thermal instability and a device using the method.
2. Description of the Related Art
Demand for highly integrated, nonvolatile miniature recording media has increased in response to the demands for compact products such as portable communication devices, electronic notebooks, or the like. It is not easy for existing hard discs to be made compact, and it is difficult to highly integrate flash memories. Thus, information storing apparatuses and methods using scanning probes have been studied.
Probes are used for several scanning probe microscopy (SPM) techniques. For example, probes are used in a scanning transmission microscope (STM), which reproduces information obtained by detecting a current with respect to differences in a voltage applied between a probe and a sample, an atomic force microscope (AFM) using an atomic force between a probe and a sample, a magnetic force microscope (MFM) using a magnetic force between a magnetic field of a sample and a magnetized probe, a scanning near-field optical microscope (SNOM) improving a limit of resolution caused by a wavelength of visible rays, an electrostatic force microscope (EFM) using an electrostatic force between a sample and a probe, and the like,
Lim, Geunbae et al. have suggested an electric field effect probe detecting surface charges of a medium using an electric field (Refer to U.S. Pat. No. 6,521,921). The suggested electric field effect probe has an electric field effect transistor type semiconductor tip forming a carrier channel using an electric field effect. Here, an electric field applied to the semiconductor tip is formed by charges or dipole moments trapped on the surface of a medium. If charges trapped on a disc form an electric field having an intensity larger than or equal to a threshold electric field intensity in correspondence with recorded information, a channel is formed, and thus the resistance of the electric field effect probe becomes low. As a result, recorded information can be reproduced according to variations in resistance corresponding to the recorded information.
Park, Hong-sik et al. have suggested a resistive semiconductor probe having a semiconductor tip with a slightly doped channel area (Refer to U.S. Publication No. 2005/0231225A1). The semiconductor tip of the resistive semiconductor probe is slightly doped with a dopant so that a weak current flows when an electric field is not formed, and thus the resistive semiconductor probe can detect a weak electric field. In other words, in the suggested resistive semiconductor probe, the semiconductor tip has low mobility so that carriers move in a channel when an electric field is not formed. Thus, a high sensitivity can be achieved with respect to a weak electric field.
However, such a resistive semiconductor tip is sensitive to heat and thus has a resistance that greatly varies with variations in temperature. A variation in resistance caused by thermal instability is a disadvantage of the resistive semiconductor probe. In other words, an instable temperature variation of the resistive semiconductor probe causes an instable current variation, i.e., a noise current, in the resistive semiconductor tip. Such a noise current is generated by a variation in temperature and occurs regardless of an electric field. The instable temperature variation of the resistive semiconductor probe is caused by an instable variation in a gap or a contact area between a medium and the resistive semiconductor probe or a non-uniform, discontinuous discharge of heat generated by the semiconductor probe or a cantilever supporting the semiconductor probe.
The gap between the resistive semiconductor probe and the medium is required to be uniformly maintained to inhibit the instable variation in the temperature of the resistive semiconductor probe. To uniformly maintain the gap, a surface of the medium facing the resistive semiconductor probe is made very smooth. Although the degree of smoothness of the surface of the medium is maximized, sufficient, effective thermal stability cannot be secured due to the limit of the degree of smoothness. Although the gap between the medium and the resistive semiconductor probe varies within a range of several nm, a noise current is generated due to variations in temperature. Also, although the surface of the medium is smooth like a mirror, the degree of smoothness of the surface of the medium cannot be adjusted within the range of several nm. As another method, the gap between the resistive semiconductor probe and the medium may be made sufficiently large. However, since a resistive semiconductor probe having a high aspect ratio is difficult to manufacture, the possibility of the gap being large is low. Thus, a noise current may be generated by thermal instability in the resistive semiconductor probe suggested by Lim, Geunbae et al.
Accordingly, in order to effectively read information recorded by charges from a medium using a semiconductor probe in which a flow of current is controlled by an electric field effect, a method of effectively reproducing a signal by improving a signal-to-noise ratio (SNR) or the like in spite of a noise current generated by thermal instability of the semiconductor probe is required.